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How to Build a Time Machine · Star Trails: From Backyard Astronomy to Cosmic Wonder

0:07
Howdy Stargazers and welcome to the season finale
0:10
of Star Trails. My name is Drew and I'll be your
0:14
guide to the night sky for the week of November
0:16
30th to December 6th. This week we're building
0:21
a time machine, or at least looking at the science
0:25
that it might take to create one. Along the way,
0:28
we'll look at some famous time machines in pop
0:30
culture, plow through Einstein's two theories
0:34
of relativity, which provide us with the equations
0:37
to calculate our time travel efforts, and we'll
0:40
verge into some wild theoretical territory that
0:44
reads more like science fiction than fact. The
0:47
winter's night sky is nearly upon us and in the
0:50
second half of the show I'll preview what you
0:53
can expect to see in the night sky for the next
0:56
month while we're on a holiday break. Whether
0:59
you're tuning in from the backyard or the balcony,
1:02
I'm glad you're here. So grab a comfortable spot
1:04
and let's see what the universe holds for us
1:07
this week. As a kid growing up in the 80s, it's
1:17
safe to say time travel movies were some of my
1:20
favorites. There was Back to the Future, which
1:23
taught us about the dangers of creating a paradox
1:26
by going back in time. There was Star Trek IV,
1:30
The Voyage Home, which showed us we could go
1:32
back in time by slingshotting around the sun
1:35
in a Klingon battle cruiser. Oh, and that humpback
1:39
whales are apparently an intelligent alien species.
1:43
Who knew? In middle school we read the original,
1:47
The Time Machine by H .G. Wells, and I also became
1:51
a fan of the 1960 film of the same name. That's
1:55
the one where Rod Taylor manages to go forward
1:58
in time some 800 ,000 years in a brassed out
2:02
steampunk sled. Once he gets there, he falls
2:05
in love with an Eloi girl and presumably brings
2:09
her back home to civilized Victorian era London.
2:12
Talk about a goal to aspire to. And later, as
2:16
a teen, Bill and Ted's excellent adventure. A
2:19
gonzo trip through time with two California goofs
2:23
who gain access to a time machine to help them
2:26
write a history term paper, of all things. I
2:29
also have to mention Disney's Star Wars knockoff,
2:33
The Black Hole. Not because there was time travel,
2:36
but because of its willful ignorance of general
2:39
relativity. More on that in a moment. The 1980s,
2:45
especially as a child, felt like an era where
2:48
anything was possible if you threw enough science
2:50
at it, especially with the rise of computers
2:53
and advancements in space travel, like the space
2:56
shuttle. Magazines of that era were wild. As
3:01
a kid, I read Omni, Discover, Astronomy, Popular
3:05
Science, and more. And one day, after reading
3:08
an article about the possibility of time travel,
3:11
I decided to grab a pack of graph paper and start
3:14
designing my own interstellar ship. One that
3:17
used theoretical propulsion that could either
3:20
take us to the speeds nearing that of light itself,
3:24
or at least provide us with the means to reach
3:26
the event horizon of a black hole. I was so naive,
3:31
but I had the science to back it up. Reading
3:34
those articles in Omnia and Discover taught me
3:37
a few important things. One, you can't go back
3:41
in time, only forward. Two, if you could travel
3:45
near the speed of light, you can travel forward
3:48
faster in time. Three, if you could orbit a black
3:52
hole, you could go even further forward in time.
3:55
Sounds pretty easy, right? These time travel
3:59
parameters were already old news even in the
4:02
1980s. See, Albert Einstein figured this stuff
4:06
out back in the 1930s. He was Doc Brown way before
4:15
Michael J. Fox jumped into a DeLorean and fired
4:18
up the flux capacitor. So in this episode, we'll
4:22
examine the two surefire ways to dilate time
4:26
based on Einstein's special theory of relativity
4:30
and his general theory of relativity. The former
4:34
deals with high speeds. The latter deals with
4:37
regions of massive gravity. We know these principles
4:41
are accurate because we use technology linked
4:43
to it every day. Those GPS satellites that float
4:48
overhead that we use for navigation, they're
4:51
time travelers in a sense. They orbit fast and
4:55
high above our planet. So much that if we don't
4:58
factor in their time offsets using super accurate
5:01
atomic clocks, we'd be sent off course down here
5:05
on Earth. So if the science actually works, what
5:09
would it realistically take to make a time machine?
5:13
One we can ride around in and fly forward ages
5:16
into the future. Let's find out. Before we get
5:24
any ideas of going back in time to play with
5:27
dinosaurs, assassinate Hitler, or play Johnny
5:30
B. Goode at a prom to make our parents fall in
5:33
love, we need to know that time is a one -way
5:36
street. We're not going back in time, sadly.
5:40
Time only moves forward. This isn't philosophy,
5:44
it's physics. The arrow of time points toward
5:48
increasing entropy, toward disorder. toward future
5:51
moments that haven't happened yet. You age forward.
5:56
Your memories accumulate forward. The universe
5:59
expands forward. Einstein's equations might be
6:04
symmetrical, but everything we observe in nature
6:06
has a preferred direction. Backwards simply doesn't
6:11
work. Yes, there are some theories where time
6:14
may go backwards, but as you'll find out later,
6:17
all these theories generally have a giant loophole
6:20
that breaks them. The big takeaway is simply
6:23
this. The rate at which time flows is flexible.
6:28
You can stretch it. You can compress it. And
6:31
if you're clever, you can make your journey into
6:33
the future faster. That's where relativity comes
6:37
in. Special relativity tells us that the faster
6:43
you move, the slower your clock ticks relative
6:47
to someone staying still. Go fast enough, really
6:51
fast, and your timeline becomes syrupy. A five
6:55
-year journey for you might be decades for the
6:58
people left on Earth. The problem is the speeds
7:01
required are insane. Take the Parker Solar Probe,
7:06
for instance, the fastest object humans have
7:09
ever built. It falls toward the Sun at about
7:13
.058 % the speed of light. That's blistering
7:18
for us and virtually meaningless for space -time.
7:23
After years of flying, the Parker Probe has accumulated
7:26
about as much time dilation as you'd get from
7:29
a long nap. To get dramatic effects, the kind
7:38
that movies show, you need to be going a significant
7:42
fraction of the speed of light. 50 to 99 percent.
7:52
Right now, we don't have any technology that
7:55
could even begin to approach those numbers. Traditional
7:58
rockets forget about it. One proposed method
8:01
that's been theorized since the 1960s involves
8:04
detonating a series of nuclear bombs off behind
8:08
your ship and riding the shockwave. That was
8:12
Project Orion, and I mention it because it could
8:15
get us in the theoretical range of 5 % of the
8:18
speed of light. This project was a real government
8:23
-funded proposal decades ago, with the idea being
8:26
we could travel to Mars in weeks and stars in
8:30
decades. This was published everywhere in science
8:34
magazines during the early space age and it echoed
8:37
through sci -fi art and articles for years after.
8:41
Omni especially adored Project Orion because
8:45
it embodied that mid -century utopian futurism.
8:48
What if we used nuclear bombs to explore the
8:51
stars? And even physicists like Freeman Dyson
8:55
worked on it seriously. Strangely, this is the
9:00
propulsion method used by my middle school graph
9:03
paper time machine. Evidently, I read about Project
9:07
Orion somewhere. I also augmented my design with
9:12
something called a Bussard ramjet. This was another
9:16
theoretical propulsion scheme that involved a
9:18
giant scoop at the front of a starship to collect
9:21
wandering hydrogen, convert it to energy via
9:24
fusion, and propel the ship forward using the
9:27
nuclear exhaust. Sounds great, right? Infinite
9:31
fuel for a long journey. Heck, even Star Trek
9:35
borrowed this idea once. The problem is, interstellar
9:39
space contains hydrogen, but just barely. The
9:42
typical density is around 0 .1 atoms per cubic
9:46
centimeter. So if we do some math, we learn that
9:50
the Boussard scoop would need to be tens of thousands
9:53
of kilometers wide, maybe millions for the concept
9:56
to work. It's not practical at any scale. And
10:00
another problem arises. Eventually, this massive
10:04
scoop would encounter a certain amount of drag
10:07
as it rams into these hydrogen atoms, becoming
10:10
more like a parachute than a stellar supercharger.
10:14
So, assuming we managed to get our Project Orion
10:18
engine fired up and we could achieve 5 % of the
10:22
speed of light, what does that really get us?
10:25
Let's set our sails for Proxima Centauri, the
10:28
closest star to us outside of our own solar system.
10:32
Because if we're pushing towards the speed of
10:34
light, we might as well go somewhere interesting.
10:40
Proxima Centauri is 4 .24 light years away. Cruising
10:45
at 5 % the speed of light, it would take nearly
10:49
85 years to get there, one way. We're going to
10:54
use something called the Lorentz factor to calculate
10:57
the time dilation. This equation tells us that
11:01
if it takes us 84 .8 years to get to Proxima
11:05
Centauri, we'd only experience about 39 less
11:08
days than clocks back on Earth. So at 5 % of
11:15
light speed, you're doing something insanely
11:18
fast by any human standard. And relativity basically
11:21
says, cool story, here's a one month bonus. Time
11:26
is stubborn until you get really close to the
11:28
speed of light. And that is some sad astrophysics.
11:36
To make this fun, let's assume we can move at
11:40
99 % of the speed of light. Because remember,
11:44
we can never achieve the speed of light itself.
11:47
The journey to Proxima would still take 4 .28
11:50
years, but things get more intriguing when we
11:53
plug that into the Lorentz equation. At this
11:56
rate, we're about 4 .5 years younger than people
11:59
back home. And because our local time is slowing
12:02
down, Our one -way journey feels more like seven
12:05
months. If you could achieve 99 .999 % of the
12:12
speed of light, the effect would even be wilder.
12:15
At that speed, Earth would see the trip take
12:17
just over four years, but from our perspective
12:20
on the ship, it would only take about a week.
12:23
So a week for us and four years for everyone
12:27
back home. do a round trip, and Earth ages eight
12:30
and a half years while we age about two weeks.
12:34
At that point, you're not just traveling through
12:37
space, you're surfing along the edge of time
12:39
itself. So speed is one way to bend time, but
12:49
it's not the easiest way. Gravity does something
12:53
extraordinary. It slows down time. The deeper
12:57
you fall into a gravitational well, the slower
13:00
your clock runs compared to someone farther away.
13:05
This means that technically your head ages faster
13:08
than your feet, really. Sadly, nothing in our
13:12
solar system is massive enough to give you sci
13:15
-fi levels of time dilation. If you orbit the
13:19
Sun closely for a year, you might gain a couple
13:22
seconds compared to interstellar space, and you'd
13:26
probably fry. Orbit Jupiter and you might gain
13:29
a few microseconds. Real gravitational time machines
13:34
need something stronger, preferably a black hole
13:38
or a neutron star, where we can achieve substantial
13:41
dilation, minutes to hours in a day. But let's
13:46
get crazy. Let's journey to the center of the
13:49
Milky Way to the supermassive black hole there,
13:52
Sagittarius A. Here we can seriously mess with
13:56
time. Sagittarius A is four million times the
14:11
mass of the Sun. It curves space -time so violently
14:15
that time practically sticks to it like glue.
14:19
If we park our spacecraft just outside the innermost
14:22
stable circular orbit, a place where the tidal
14:25
forces won't tear us apart, our personal clocks
14:29
would slow dramatically compared to clocks back
14:31
on Earth. We calculate it using the Schwarzschild
14:35
time -dilation equation. Depending on the specific
14:40
orbit, one hour for me might equal days, weeks,
14:44
or even years back home. If you've seen the movie
14:47
Interstellar, you've seen this type of dilation
14:50
accurately depicted when the crew momentarily
14:53
landed on Miller's planet, then returned to their
14:56
ship to find years had elapsed for the scientists
14:59
who remained behind. Of course, there's one tiny
15:03
detail. Sagittarius A is 26 ,000 light years
15:08
away. Getting to the thing that lets you time
15:11
travel efficiently requires a level of time travel
15:14
we don't have. Remember when I mentioned Disney's
15:21
Black Hole movie earlier? Well, what bugs me
15:24
about that film is that they're orbiting right
15:26
at the edge of a black hole, seemingly ignorant
15:29
to its effects on time. Remember, Einstein worked
15:33
this out nearly 50 years earlier. Not sure it
15:36
matters because they all fall into the black
15:38
hole at the end of the film and the bad guys
15:41
emerge in some existential version of hell and
15:44
the protagonists pop out near a nice -looking
15:46
planet, presumably to repopulate Eden or something.
15:56
So why does this happen? Why does moving fast
15:59
or floating in a gravity well affect time? To
16:04
the universe, time isn't an independent thing.
16:07
Time is woven into space. It's literally part
16:11
of the geometry. When you move or when gravity
16:14
bends space -time, you're changing that geometry.
16:19
Your path through space -time gets stretched
16:21
or squeezed and your clock ticks along that path.
16:25
When you approach the speed of light, you're
16:28
using more of your motion budget to move through
16:31
space and less to move through time. Relativity
16:35
says there's a fixed speed you move through space
16:38
-time, always the speed of light, but divided
16:41
between space and time. If you move faster through
16:45
space, you must move slower through time. So
16:50
your personal time that is the time your body
16:53
experiences slows down But you don't feel it
16:56
to you everything feels normal your heart beats
16:59
normally your brain fires normally It's only
17:02
when you compare with someone who stayed behind
17:05
that you see the difference Gravity slows time
17:12
because it bends space -time Remember, general
17:16
relativity redefined how science sees gravity.
17:19
It isn't just a force pulling downward. It's
17:22
a curvature of space -time itself. The closer
17:26
you are to a massive object, the more space -time
17:29
is bent. And the deeper you sit in that curvature,
17:32
the slower time passes for you. That's why clocks
17:36
tick slower at sea level than on mountaintops.
17:40
It's why GPS satellites tick faster than your
17:43
phone. And near a black hole, space -time is
17:46
curved so violently that time practically congeals.
17:50
Even light struggles to escape. We understand
18:02
the how. The equations match experiments perfectly.
18:06
GPS, particle accelerators, atomic clocks flown
18:10
on planes, or nuclear -powered clocks on mountaintops,
18:14
and so on. But the deeper why is more philosophical.
18:19
The best answer physics gives us is because space
18:22
-time has geometry, and time is one of its dimensions.
18:27
Einstein didn't explain time dilation so much
18:30
as he showed that it's a natural consequence
18:33
of living in a four -dimensional universe, where
18:36
the speed of light is constant and mass bends
18:39
space -time. Once you accept those two facts,
18:43
everything else follows. Time slows near gravity.
18:47
Time slows at high speeds. It's all a form of
18:50
geometry. We don't know why the universe has
18:54
this geometry instead of some other one, but
18:57
we know that once you plug the geometry in, everything
19:00
behaves exactly as expected. So what use is a
19:05
time machine if we can't go back in time? There
19:08
are theories that enable this, but so far quantum
19:12
physics has sort of put its foot down. In the
19:15
last 15 or 20 years, physicists and quantum information
19:19
theorists have been exploring what happens if
19:22
you allow quantum bits to interact with versions
19:25
of themselves from the future, along a so -called
19:28
closed time -like curve. This isn't a proposed
19:33
machine, more of a thought experiment to test
19:36
the boundaries of causality, and the punchline
19:39
is always the same. The quantum math protects
19:43
itself from paradox. If you try to send contradictory
19:47
information into your own past, the equations
19:50
smooth it out. so no paradox forms. It's like
19:54
the universe is auto -correcting itself. Even
19:58
at the smallest scales, the quantum world, the
20:01
universe seems allergic to backward time travel.
20:06
One modern idea, the Novikov self -consistency
20:10
principle, says that even if you could go backward
20:13
in time, the universe wouldn't let you create
20:16
a paradox. Every choice you try to make in the
20:19
past loops around to become exactly the thing
20:22
that made time travel possible in the first place.
20:26
It's cosmic predestination. The past is locked,
20:30
the future is open, and time refuses to contradict
20:34
itself. Another modern quantum theory leaves
20:39
a tiny conceptual crack open. You might not be
20:42
able to visit your own past, but you could, in
20:45
theory, visit the past of a different branch
20:49
of the universe, one where your arrival doesn't
20:52
break causality. It's not time travel in the
20:55
traditional sense. It's more like switching tracks
20:58
on a cosmic railroad made of probability. So
21:02
far, this idea has been untestable. I can't imagine
21:06
why. For those of us stuck down here on Earth,
21:11
the best time travel we can do is simply observe
21:14
the universe. Feel like I've been saying this
21:17
a lot lately, but every time we look up we're
21:20
looking back into the past Sometimes the recent
21:23
past like the light leaving Jupiter is 40 minutes
21:27
old Light from our Sun is eight minutes old Sometimes
21:32
we look at the deep past like the ancient glow
21:35
of galaxies whose first stars fired long before
21:39
Earth even formed The night sky is a map of moments
21:43
that have already happened, still streaming towards
21:46
us across the vastness. We may never build a
21:50
starship that reaches 99 .999 % of the speed
21:54
of light. We may never orbit a supermassive black
21:58
hole and return home decades ahead of the people
22:01
we left behind. And we certainly won't be sitting
22:05
on Victorian chairs with bejeweled levers racing
22:08
off to fight the Morlocks, ducking into a police
22:12
call box or driving a DeLorean like we stole
22:15
it to rewrite history. It's not about a machine
22:20
so much as a perspective. Relativity teaches
22:23
us that time isn't a singular river, but more
22:26
like thousands of branching streams. And when
22:29
speed or gravity gets involved, those streams
22:32
can meander. Ultimately, they still all run forward.
22:38
So if you really want to experience time travel,
22:41
go outside tonight. Look up. Find a star. The
22:45
moment you're seeing left that star years ago,
22:49
sometimes centuries, sometimes millions or billions
22:52
of years ago. You're reaching backward with nothing
22:55
but your eyes and the physics of the cosmos.
22:59
those are ancient photons from the past hitting
23:02
our retinas in the present. In the second part
23:15
of this episode, we'll explore what's going on
23:18
overhead for the month of December. That's coming
23:21
up after the break. Stay with us. Welcome back.
23:38
Star Trails is taking a breather in December,
23:41
but that doesn't mean the sky is taking a month
23:44
off. Far from it. December is one of the richest
23:47
observing months of the entire year. So here's
23:52
your December sky strategy. Think of it as a
23:55
guidebook for the whole month. Three simple beats
23:58
to follow. Bright moon season, meteor season,
24:02
and deep winter season. December starts with
24:06
a burst of brightness. We get a full supermoon
24:09
on December the 4th, the final supermoon of the
24:13
year, traditionally called the cold moon. It'll
24:16
look big and bold, climbing the eastern sky just
24:19
after sunset. A week later, on December 11th,
24:23
the moon hits third quarter and lights up the
24:26
sky. But stick with it, because the sky gets
24:29
wonderfully dark by December 19th, when we hit
24:32
the new moon. The new moon window will be your
24:35
best opportunity for deep sky observing. And
24:39
then the month closes out with a first quarter
24:41
moon on December 27th, a bright half moon riding
24:45
high in the afternoon and evening sky. December
24:49
also gives us two meteor showers. One is spectacular.
24:53
The other is subtle, but atmospheric. First,
24:57
the Geminids. They're active from December 4th
25:00
through the 20th, and they peak on the night
25:02
of December 13th, into the early morning of the
25:05
14th. The moon will be just a thin waning crescent
25:09
rising late so the sky will be beautifully dark.
25:12
Under good conditions, the Geminids can deliver
25:15
more than 100 meteors per hour. If you're going
25:19
to commit to one night this month, make it Geminid
25:22
night. Face east, get comfortable, and start
25:25
watching around 10 p .m. The ursids are the second
25:30
meteor shower of the month. These are active
25:32
from December 13th through the 26th, and they
25:36
peak right around the time of the solstice. The
25:39
ursids are gentle, maybe five to ten meteors
25:42
an hour. Look north toward the Little Dipper,
25:45
and let the longest night of the year do the
25:48
rest. December's planetary lineup is straightforward.
25:53
Mercury is the prize for early birds. It has
25:56
one of the best morning showings of the entire
25:59
year. Look low in the southeast about 45 minutes
26:03
before sunrise from December 1st through the
26:06
23rd with the greatest visibility around December
26:09
7th. Jupiter is the showstopper of the month.
26:13
Bright, obvious, and unmistakable in Gemini in
26:17
the early evening. If you have a telescope, Jupiter
26:20
is your December anchor. Belts, zones, storms,
26:25
and the dance of its four big moons every single
26:28
night. Saturn hangs in the southwestern sky during
26:32
the early evening hours. It's fading, sinking
26:35
earlier each night, but still makes a wonderful
26:38
warm -up target in Aquarius before you turn your
26:40
attention to the winter constellations. Venus
26:44
and Mars are both tucked too close to the Sun
26:47
this month. If you're hunting planets, your trio
26:50
is Mercury at Dawn, Saturn at dusk and Jupiter
26:53
all night. Now for the real magic, the winter
26:57
sky itself. By 9 or 10 p .m., the whole eastern
27:01
sky is glowing with the constellations that define
27:04
the season. There's Orion rising with his belt
27:08
leading downward towards Sirius and Canis Major,
27:11
the brightest star in the night sky. Above Orion
27:15
is Auriga with brilliant Capella. To the left
27:18
are Castor and Pollux in Gemini. overhead, drifting
27:23
westward, the Pleiades sparkle like a tiny jewel
27:26
box. If you want to keep things simple during
27:29
December, here's an easy plan. Session 1, early
27:34
December around the full supermoon. Watch the
27:37
cold moon rise. Use its glow to trace the winter
27:40
constellations. Let Jupiter and Saturn orient
27:43
you in the evening sky. Session 2, the Geminid
27:48
weekend, December 13th and 14th. Block out one
27:52
night, go somewhere dark if you can, bundle up,
27:56
lean back, and let the meteors pour out of Gemini.
28:00
Session 3 Solstice night, December 21st and 22nd.
28:05
Take advantage of the new moon darkness. Check
28:08
out the Orion Nebula, the Pleiades, the clusters
28:11
in Auriga, and then, after midnight, turn north
28:15
for the soft drizzle of the Ursids on the longest
28:18
night of the year. If you manage even one of
28:21
these sessions, you'll have a memorable December.
28:25
We'll see you again in January. I hope you all
28:27
have a great holiday and maybe some of you will
28:30
unwrap a new pair of binoculars or a smart scope.
28:34
If you do, let me know. If you found this episode
28:40
interesting, please share it with a friend who
28:42
might enjoy it. The easiest way to do that is
28:44
by sending folks to our website, StarTrails .Show.
28:49
And if you want to support the show, use the
28:51
link on the site to buy me a coffee. It really
28:54
helps. Be sure to follow Star Trails on Blue
28:58
Sky and YouTube. Links are in the show notes.
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Until we meet again beneath the stars, be excellent
29:04
to each other. And party on, dudes!